Add session validation UI and expose isValidating in auth context

- LoginPage now shows a loading spinner while validating the session
- Redirect to dashboard automatically once validation completes and session is valid
- Expose isValidating state through AuthProvider for components to track validation status
- Update useAuth hook to return isValidating along with isAuthenticated

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/pages/LoginPage.tsx

@@ -6,7 +6,7 @@
  * parameter) or the dashboard.
  */
 
-import { useState } from "react";
+import { useState, useEffect } from "react";
 import {
   Button,
   Field,
@@ -75,7 +75,7 @@ export function LoginPage(): React.JSX.Element {
   const styles = useStyles();
   const navigate = useNavigate();
   const [searchParams] = useSearchParams();
-  const { login } = useAuth();
+  const { login, isAuthenticated, isValidating } = useAuth();
 
   const [password, setPassword] = useState("");
   const [error, setError] = useState<string | null>(null);
@@ -84,6 +84,33 @@ export function LoginPage(): React.JSX.Element {
   const next = searchParams.get("next") ?? "";
   const safePath = /^\/(?!\/)/.test(next) ? next : "/";
 
+  // Redirect to dashboard if session is valid and validation is complete
+  useEffect(() => {
+    if (isAuthenticated && !isValidating) {
+      navigate(safePath, { replace: true });
+    }
+  }, [isAuthenticated, isValidating, navigate, safePath]);
+
+  // Show loading state while validating session
+  if (isValidating) {
+    return (
+      <div className={styles.root}>
+        <div className={styles.card}>
+          <div
+            style={{
+              display: "flex",
+              alignItems: "center",
+              justifyContent: "center",
+              minHeight: "200px",
+            }}
+          >
+            <Spinner size="large" label="Checking session…" />
+          </div>
+        </div>
+      </div>
+    );
+  }
+
   function handlePasswordChange(ev: ChangeEvent<HTMLInputElement>): void {
     setPassword(ev.target.value);
     setError(null);

frontend/src/providers/AuthProvider.tsx

@@ -62,6 +62,8 @@ import { useSessionValidation } from "../hooks/useSessionValidation";
 export interface AuthContextValue {
   /** `true` when the backend considers the session valid. */
   isAuthenticated: boolean;
+  /** `true` while the session is being validated on app mount. */
+  isValidating: boolean;
   /**
    * Authenticate with the master password.
    * Throws an `ApiError` on failure.
@@ -216,8 +218,8 @@ export function AuthProvider({
   }, []);
 
   const value = useMemo<AuthContextValue>(
-    () => ({ isAuthenticated, login, logout }),
-    [isAuthenticated, login, logout],
+    () => ({ isAuthenticated, isValidating, login, logout }),
+    [isAuthenticated, isValidating, login, logout],
   );
 
   // Show loading spinner while validating session on mount.